A sensor that detects various kinds of physical amounts such as temperature, pressure, or a flow rate is used in a vehicle, a consumer appliance, and industrial equipment for control or securing safety. Such a sensor is configured by a physical amount detecting unit that extracts a physical amount as an electric signal, an electrical signal processing unit that amplifies the electric signal to a desired amplitude, an output signal modifying unit that outputs the detected physical amount to the outside, and the like. The output signal is input to an apparatus that reads a signal of the sensors and performs a certain kind of control or display and the like. Since such an apparatus changes an operation or a display thereof based on information from the sensor, an output signal of the sensor should not cause physical amount information to deteriorate in signal transmission or should not increase electromagnetic noises caused by the transmission of the signal and the like, in order to obtain high reliability.
Meanwhile, recently, there is a case where a calculation circuit is used in a part or all of the electric signal processing unit or the output signal modulating unit. In this case, it is reasonable that a modulation technique by a pulse signal that can be easily handled by the calculation circuit is used for the output signal of the sensor. Though the modulation by a pulse signal includes frequency modulation, PWM modulation, PCM modulation, and the like, any of these uses binary logic or time between pulses.
Since such a signal does not have a median level, information transmission is highly accurate, but electric noises are likely to increase due to a steep change of a signal level. Therefore, a through rate control technique in which logic transition speed of the pulse signal is properly smoothed is used.
For example, a technique for such control is disclosed in PTL 1.
PTL 1 discloses a circuit example in which logic transition speed of a digital signal is smoothed, and an original digital value is input to an on/off generator 9 and causes a transistor Q1 to be turned on and also a transistor Q3 by an ON signal. Series resistors R1 and R2 are connected to a circuit switched by the transistor Q3 and a capacitor C is connected to a connecting point of the resistors R1 and R2. Therefore, a potential at an output signal drawing point a is continuously and gradually increased based on time constants of the resistor R1 and the capacitor C. In this manner, adjustment of logic transition speed with time constants of a resistor and a capacitor is widely performed. If a plurality of capacitors and resistors are prepared by using such a circuit and switching is performed by a selecting switch (such as a transistor), through rate control can be arbitrarily performed.
Further, PTL 2 discloses a technique of adjusting logic transition speed only with a transistor without using a passive component such as a resistor or a capacitor.
PTL 2 discloses that an output control circuit 3 that drives transistors 11 and 12 of an output unit has a function for controlling electric current for driving the transistors 11 and 12. In specific, as illustrated in FIG. 2, transistor groups MN1 to MNn that have different electric current driving capabilities are provided, the respective transistor groups have independent signals TA1 to TAn for driving transistor groups MN1 to MNn, and selectively use the signals TA1 to TAn in order to provide a desired logic transition speed to an OUT terminal of FIG. 1. Since each of the transistor groups has a different electric current driving capability, continuous and smooth driving signals can be obtained depending on selection of the transistor groups, thereby functioning as through rate control.